1. Field of the Invention
The present invention relates to a method for realizing a transmit focusing in an ultrasonic imaging system, and more particularly, to a method for realizing a transmit focusing with respect to all imaging points by synthesizing pulse patterned plane waves having the same imaging point using transmit data and receive data with respect to plane waves of each pulse pattern having a different travelling direction and a linear time delay.
2. Description of the Related Art
A transmit focusing in an ultrasonic imaging system is accomplished by linearly overlapping plane waves having a different travelling direction in an imaging depth. A transmit sound field realizing the transmit focusing is accomplished from spacial expansion of plane waves passing through an imaging depth. Realizing a transmit focusing will be described with reference to the accompanying drawings.
FIG. 1 shows a continuous plane wave whose directional angle is .theta.. In FIG. 1, a linear transducer is positioned on the x-axis around the origin on the x-z coordinate. A continuous plane wave .PHI..sub..beta. which is transmitted from each element of the linear transducer and proceeds at a directional angle .theta. with respect to the z-axis is expressed as the following equation (1).
.PHI..sub..beta. =e.sup.-i.omega.t e.sup.ik.beta.x e.sup.ik.upsilon.(z-z.sup..sub.f .sup.) (1)
Here, .beta.=sin .theta., .upsilon..sup.2 +.beta..sup.2 =1, and k=.omega..sub.0 /c.upsilon.. Also, .omega. is a frequency, t is the time when a continuous plane wave reaches any of imaging points (x, z), .omega..sub.0 is a frequency at .theta.=0, c is a velocity of the continuous plane wave, and z.sub.f is a focal point.
FIG. 2 shows overlapping continuous plane waves (.PHI..sub.0, .PHI..sub..beta.) whose frequencies are same and whose directional angles are 0 and .theta., respectively. In FIG. 2, the largest signal intensity appears at a focal point (z=z.sub.f) where phases of the two continuous plane waves are overlapped. Also, as it is farther from the focal point toward the lateral direction, the phase difference of the two continuous plane waves becomes larger, and the signal intensity becomes small at the point to the receding lateral direction.
Thus, if all continuous plane waves which travel within the directional angle of the lateral direction from the focal point, that is, .+-..theta..sub.m (m is an integer) are overlapped, the overlapped wave is expressed as the following equation (2). ##EQU1##
As shown in the equation (2), the overlapped wave with respect to the lateral direction continuous plane waves has the characteristics of the sinc function. That is, it can be seen that the transmit sound field due to overlapping the lateral direction continuous plane waves possesses the characteristics of the sine function.
Also, as shown in FIG. 2, it can be seen that the phases of the two continuous plane waves are not consistent at an imaging depth beyond the focal point z.sub.f on the center axis having a main lobe. Thus, a signal intensity becomes small at the imaging depth beyond the focal point. Finally, the overlapped wave of the equation (2) has the diffractive characteristics according to the imaging depth. This is the reason why the transmit sound field according to the imaging depth is represented as a function of .beta..sub.m.
Accordingly, if each frequency .omega. of continuous plane waves having a different travelling direction and a frequency .omega..sub.0 at .theta.=0 has the relationship of .omega.=.omega..sub.0 /.upsilon., the phases of the continuous plane waves are consistent at all the imaging depths on the central axis having a main lobe. The phase consistent continuous plane waves on the central axis having the main lobe are shown in FIG. 3. The equation (2) is expressed again as the following equation (3). ##EQU2##
The equation (3) has a perfect non-diffraction characteristic in which the functions of x and z are separated. Also, the overlapped continuous plane wave of FIG. 3 should have a resolution of an imaging depth direction in order to be applied as a medical ultrasonic wave. In order to make the continuous plane wave have a resolution of the imaging depth direction, the pulse patterned plane waves having various frequencies should be overlapped. Thus, the equation (3) is frequency integrated and expressed as the following equation (4). EQU .PSI.(x,z,t)=.intg..sub.BW F(.omega.).PHI..sub..beta. (x,z,t)d.omega. (4)
Here, .PSI.(x, z, t) represents a frequency integrated overlapped wave, BW does a frequency band of a pulse, and F(.omega.) does a frequency characteristic function with respect to a pulse transmission and reception system.
A pulse patterned plane wave which is transmitted for each element of the linear transducer is obtained by substituting the equation (4) by z=0. However, since the pulse patterned plane wave obtained by substituting the equation (4) by z=0 has the infinite length on the temporal axis, a truncation error is induced. Also, the pulse pattern is complicated. Thus, in order to realize the pulse, complicated hardware is needed.
In addition, the lateral direction resolution of the pulse patterned plane wave relies on .beta..sub.m. If .beta..sub.m is increased to enhance a resolution, a non-diffraction characteristic is weakened. Also, an overall signal intensity during transmit focusing with respect to all imaging points is smaller than that with respect to one point, a signal-to-noise ratio is lowered.